Among the following which acts as the strongest acid in reaction where nitric acid behaves like a base is ____ ?

Following is the reaction

$HN{O}_3+HF\to F_2+H_{2}O+NOF$

As a non-aqueous solvent HF ionises in the following manner more easily as compared to other hydra acids.

$2HF\leftrightharpoons H{F_2}^{-}+H^{+}$

Due the ease of the above step, nitric acid can be protonated easily and so it can act as a base.